The four cycle Otto engine was originally throttled by retarding the spark advance; however, this method of throttling was very inefficient and was replaced with today's method of using a butterfly carburetor valve. During the initial development of the automobile, the engine had a low compression ratio and was under-powered, compared to the weight of the vehicle. These early automobiles used nearly full power when cruising down the road and relied on the engine throttle primarily to control the engine during city traffic conditions.
During the subsequent evolution of the automobile, the engines became more powerful relative to the weight of the automobile, and the automobile became more streamlined so that less work was required to propel a given weight. With more power per weight, the automobile now had the ability to accelerate very quickly and provide the agility needed in traffic conditions.
The development of automobiles with today's power-to-weight ratio created a condition whereby the engine was throttled to produce only ten percent to twenty percent of its full power when cruising at 55 MPH. During cruising conditions when the engine is throttled to fifteen percent of full power, the efficiency of the engine drops from around thirty percent to half, or fifteen percent.
The inherent inefficiency of today's method of throttling automobile engines during cruising conditions has been recognized by many researchers and has led to a number of solutions.
The first method used was to replace the automobile engine with a diesel engine which is more efficient during partial loading conditions. This solution has not been very acceptable, however, because the total automobile fleet could not convert to the use of diesel fuel. In addition, there are inherent problems with the diesel engines, such as pollution, cost and lack of power available.
Another method being used is to reduce the engine size and install a turbo charger to provide the additional full load power. This method has not been readily accepted, primarily because of the lag time involved to achieve full power and/or turbine wear problems.
Still other methods of improving partial load efficiency have been to use compound engine systems that use an exhaust turbine for an engine bottoming cycle such as a Rankine steam engine which runs on exhaust heat. These methods have not proven very acceptable because of the small relative gains versus the cost and maintenance problems.
More recently, researchers have studied using solenoid-actuated valves to reduce the number of cylinders being used. This method has not been judged acceptable because of the problems of controlling the system and the increased engine wear.
Another similar method uses solenoid valves to control the intake timing. This method also produces rough engine running conditions, which produces engine wear.
The most popular methods being researched today use the engine intermittently at full power and use stored power during partial load conditions. Fly wheels and variable transmissions have been used by some researchers, while others have used batteries to store engery which powers a small electric engine. Still other researchers have used a second smaller engine, such as a motorcycle, attached to the back of the car to power the automobile during cruising conditions. These methods have accomplished drastic improvements in the operating efficiency of the automobile by operating the engine at thirty percent efficiency rather than fifteen percent during cruising conditions.
All of the known methods proposed, however, have a major limitation in that the engine life is reduced and the driver of the automobile is inconvenienced. The cost trade-off of these methods is justified, however, because the fuel savings outweigh the increased cost and inconvenience.
Where the Otto engine has been used in the development of aircraft, researchers built test stands to test the engine performance under flight conditions. The major reason for performing these tests was to determine the power available at a given altitude and to develop methods of de-icing the carburetor. These tests developed power-fuel flow data for the pilots so that they could estimate flight range and speed of travel. An example of such a test stand is in Swiss Pat. No. 199,229.
At present, most aircraft with Otto engines have a service manual which shows the power and fuel flow for given altitude conditions. These charts usually show operating conditions up to 25,000 ft. The service ceiling for most propeller-driven aircraft is 25,000 ft, at which point the air density is about half that at sea level. At this altitude, the engine can only produce half of the power available at sea level. Under these conditions, the airplane engine is operating at the same efficiency as at sea level and the fuel flow charts show that the fuel consumption per mile is the same regardless of altitude. The reason there is no apparent difference is that the efficiency of a throttled engine starts to decrease at the half-throttled condition.
Pilots of today's jet aircraft are not limited to the 25,000 ft. service ceiling because the aircraft are not propeller driven and have more power available. The propeller is limited in altitude because as the air gets thinner, the revolutions per minute of the propeller cannot be increased (Mach I tip speed limit). The attack angle of the propeller is therefore increased up to a near stall condition at the service ceiling.
With the introduction of jet aircraft, many pilots now fly up to a service ceiling of 41,000 ft. These pilots are equipped with service manuals which show fuel flow per mile travelled. There is unanimous agreement among these pilots to get to altitude quickly and stay at altitude to conserve fuel. At the service ceiling, they are flying at about twenty-five percent of full power at sea level. There is a general misconception among these pilots as to why they get less fuel consumption at altitude. Most believe that the air is thinner and so the drag is reduced. In actuality, the lift-to-drag ratio is constant for a given attack angle and velocity increases with decrease in the density of the air.
It is not obvious to most pilots of jet aircraft that, in fact, when the jet engine is operating in thinner air, the fuel-to-air ratio can be maintained high and the combustion temperature maintained high for good efficiency. If the same aircraft has to reduce its altitude and still run at twenty-five percent of full power, then the jet engine has to be run with a greater air-to-fuel ratio and this lowers the combustion temperature and the engine efficiency.
The use of altitude or reducing the density of the air is therefore a very efficient way of throttling a jet engine. The same is true for a diesel engine, in which case the combustion temperature can be maintained high during throttling rather than reducing the temperature by decreasing the fuel-to-air ratio.
It is not obvious, however, that an Otto engine can be efficiently throttled by the same means. In the Otto engine, the fuel-to-air ratio remains constant as does the combustion temperature. In the Otto engine, the inefficiency of throttling with a butterfly valve is caused by an apparent increase in inlet temperature and an apparent increase in back pressure.
If an Otto engine were operating at 41,000 ft., it would receive inlet air at 1/5 the pressure of sea level and at -60.degree. F. The engine would also exhaust to an atmosphere of 1/5 the pressure of sea level. The power produced by the Otto engine would be about twenty-five percent of full power at sea level and at nearly maximum efficiency.
If the same engine operating at 41,000 ft. were to have the inlet temperature raised to 100.degree. F., and the back pressure increased to sea level pressure, the efficiency of the engine would be drastically reduced. From this phenomenon, I have therefore deduced that the Otto engine can also be very efficiently throttled by the use of altitude or reducing the density of the air.